anged by the movement of the valve rod LN round the centre
the opening to lead is invariable. For the complete drawings'
these link motions see p. 640, where the Allan motion is' also
>wn. In that gear the alteration of lead would be only about'
If that of the Stephenson, as the linking-up is only partly'
scted by the vertical movement of the link.
P. 686. Edwards' Air Pump.—The pump is used to re-
>ve the air and condensation water in marine engines, and is of
> simplest possible character. The piston A, Fig. 1016,
.veiling within the cylinder B, has a conical under-surface that
actly fits the lower cover D, and there is no foot-valve. On
e up-stroke the condensed water trickles into the space D
rough the opening E, while the water of the previous stroke is
ted and delivered through the head valves F into the upper
.amber. On the down-stroke the piston reaches almost to the
rttom cover, as at AX, displacing the water, and causing it to
cend through the openings cx into the space above the piston.
ais action takes place both surely and quickly on account of
e free passage for air which is indicated. The improved valve
id and guard, G and j, are fixed to the top cover, so that the
:ter may be lifted and the valve H examined while the pump is